A point to point radio link will typically be used to transmit data between two stations, which may be several kilometres apart. At each station there will typically be a mast to which antennas are fixed for transmission and reception, and the height, orientation and type of antenna is chosen to minimise the signal loss in the link within practical constraints. However, atmospheric ducting effects and obstructions to the link can cause a loss of signal in some circumstances. It is beneficial to employ error correction coding, which will considerably improve the tolerance of the system to signal loss.
Typically error correction coding may consist of serially concatenated convolutional and Reed-Solomon codes. Alternatively, a method known as multilevel coding may be used, which uses a different rate of code according to the significance of the bit being protected, where the least significant bits are given more powerful protection than the more significant bits. This type of code has a lower requirement for signal to noise ratio than serially concatenated convolutional and Reed-Solomon codes when the noise has white Gaussian statistics. However, the implementation of the coding is typically complex and the latency of the resulting data link may be undesirably high. An example of multi-level coding is sequentially decoded low density parity coding.
The type of modulation employed on the link can be altered adaptively to maximise the capacity of the link given the signal to noise ratio. Generally, more spectrally efficient modulation formats require a higher signal to noise ratio to operate at an error rate which is within the error correction capabilities of the coding employed. This modulation type may be chosen by a measure of signal to noise ratio, averaged over an appropriate period of time. However, generally the selection of the appropriate modulation type is inefficient due to the need to err on the side of caution when predicting whether a more spectrally efficient scheme could be employed.